Ultrafast lasers are pulsed-lasers that deliver pulses having a duration as short as a few femtoseconds (fs). In many applications of such lasers, the shorter the pulse-duration the more effective the laser may be. Such lasers are frequently configured as master-oscillator power-amplifier (MOPA) systems in which a master laser provides a seed-pulse to an optical amplifier, usually a regenerative or multipass amplifier.
Typically the seed-laser delivers pulses having an essentially Gaussian temporal profile and an essentially Gaussian spectrum. The spectrum may have a bandwidth of between about 20 and 200 nanometers (nm). The lasers usually include mirrors that introduce negative group delay dispersion (NGDD) to light reflected therefrom. These mirrors minimize temporal broadening of the pulses due to positive group delay dispersion resulting from differences in velocity of the spectral components of the pulse.
In a paper “No loss spectral phase correction and arbitrary pulse shaping and arbitrary phase shaping of regeneratively amplified femtosecond pulses using MIIPS”, J. Pastirk, B. Resan, A. Fry and J. Mackay, OPTICS EXPRESS 9543, 14, 20, (Oct. 2, 2006), it is taught that if the Gaussian spectrum of seed-pulses is “shaped” into a non-Gaussian form, by amplitude modulation, before being delivered to the amplifier, the duration of the amplified pulses will be less than had the spectrum not been so shaped.
FIG. 1 schematically illustrates in block-diagram form, the MOPA arrangement 10 described in the Pastirk et al. paper. In this arrangement, a pulse having an essentially Gaussian spectrum is delivered from a seed laser 12 to a pulse-shaping apparatus 14. The pulse-spectrum is amplitude modulated in a pulse shaper 14, then delivered to an amplifier 16 for amplification.
FIG. 2 schematically illustrates the essentially Gaussian spectrum of a pulse from laser 12 (dashed curve) and the spectrum of the pulse after shaping in pulse-shaper 14 (solid curve) and amplification in amplifier 16, with the spectra, here, being normalized. The shaped seed-pulse has two intensity-peaks with an intensity-trough between the peaks. The effect is that the shaped spectrum has almost twice the FWHM bandwidth of the original spectrum.
FIG. 3 schematically illustrates intensity as a function of time for amplified Gaussian-spectrum (dotted curve) and shaped-spectrum (solid-curve) pulses. The amplified spectrally-shaped pulse has a FWHM duration of 27.6 femtoseconds (fs) compared with a FWHM-duration of 35.6 fs for the amplified Gaussian-spectrum pulse. The spectral shaping has no effect on the amplified pulse energy.
In the pulse-shaper, a pulse-beam from the seed-pulse laser is caused to diverge in one transverse axis, for example, by passing the beam through a dispersive device such as a prism. The divergence is due to dispersion of the spectral components of the pulse by the prism. The diverging beam (spectrum) is collimated by a lens, or a concave mirror, and directed onto a one-dimensional spatial light modulator (SLM) located at a focal-length distance of the lens from the lens. This arrangement transforms the pulse from a time-domain to a wavelength-domain. The SLM is used to shape the spectrum by selectively attenuating selected wavelength-components of the spectrum. The shaped spectrum is then transformed back to the time domain by focusing the spectrum back onto the same or a different dispersive device. These operations require a relatively complex optical system particularly if the pulse-shaper has to optimally shape pulses of different spectral bandwidths.
Inclusion of such a pulse-shaper in a MOPA laser system can add significantly to the cost of the system. Many users of ultrafast MOPA systems are institutions such as not-for-profit foundations and universities. Such institutions often have limited budgets for capital equipment. Accordingly, it would be useful for such institutions if the above-discussed advantages offered by using spectrally-shaped seed-pulses in an ultrafast MOPA system could be enjoyed without a need for a pulse-shaper.